Fuel injection pump

ABSTRACT

A fuel injection pump includes an injection onset adjustment piston in a cylinder bore that adjusts injection. The onset adjustment piston is embodied as a following piston which includes therein a control slide valve. The onset adjustment piston encloses a work chamber on one end, and is acted upon by the pressure in the work chamber counter to the force of a restoring spring. The onset adjustment piston is connected to an essentially stationary part of a cam drive of the fuel injection pump. In order to prevent feedback from the work chamber via the control slide valve and its adjustment, the control slide valve is effectively damped by means of a separate pin on an end of the control slide valve which extends into a blind bore of the onset adjustment piston.

PRIOR ART

The invention is based on a fuel injection pump for a vehicle. DE 35 32719 discloses a fuel injection pump of this kind in which the pressurefluid supply to the cylinder bore takes place via a check valve, whilethe pressure fluid discharge proceeds via the chamber that contains therestoring spring. The control slide valve end face which is subjected tothe control pressure defines the control chamber in the cylinder boreover the entire diameter of the cylinder bore, which chamber, on theother end, communicates with a control pressure inlet via a throttle.Injection timing mechanisms of this kind are very sensitive to controlpressure fluctuations and have a tendency for the control slide valve toexecute oscillating movements relative to the injection timingmechanism, which leads to an unstable work behavior of the injectiontiming mechanism. In fact, the throttle bore has the effect that kicksback on the control pressure supply end, which are produced by theoperation of the fuel injection pump, should have as little effect aspossible on the control slide valve, but a balanced, average workingpressure, which is effective in the control chamber, is onlyincompletely achieved.

ADVANTAGES OF THE INVENTION

The fuel injection pump according to the invention, has the advantageover the prior art that the control slide valve is additionally dampedin its movements so that nevertheless, pressure fluctuations occurringin the control chamber do not reach through and indirectly excite thecontrol slide valve. If the control piston is excited to oscillate bypressure surges, the fluid enclosed in the blind bore by the pin isfirst displaced by means of the throttle gap. The throttle action, whichis produced as a result, effectively suppresses the tendency of thecontrol piston to oscillate.

BRIEF DESCRIPTION OF THE DRAWING

An exemplary embodiment of the invention is shown in cross section inthe drawing and explained in detail in the ensuing description.

DESCRIPTION OF THE EXEMPLARY EMBODIMENT

Fuel injection pumps of the distributor type can be provided either aspumps with an axially driven pump piston that is used as both adistributor and a pump piston, or radial pistons can be provided, whichfeed radially into a supply conduit disposed in a distributor. FIG. 1shows a section through a so-called radial piston pump of this kind,which is of the known type. Four pump pistons 1 are provided, which aresupported so they can sealingly slide in radial bores 3 of thedistributor 2, at the same angular spacing in a common plane radial tothe axis of the distributor 2. On their one end face, they enclose acommon pump work chamber 4, which is filled with fuel via a meteringdevice in a known manner not shown in detail here during the radialoutward stroke of the pump piston 1, and during the radial inward strokeof the pump piston, communicates via a pressure line, likewise notvisible here, with a distributor opening on the jacket face of thedistributor 2, wherein the distributor opening controls injection linesthat lead from the circumference of the distributor, one of the lines isrespectively supplied with fuel that has been brought to injectionpressure when the pump piston is moved inward. Through means not shownin detail, a drive shaft drives the distributor to rotate in such a waythat on the one hand, the distributor opening can carry out its controlfunction and on the other hand, the pump piston is moved in thecircumference direction. Roller tappets 6 rest against the end disposedopposite the pump work chamber 4 and follow a cam track 7 which isdisposed on a cam ring 8, on its annular face that points inward towardthe distributor. The cam ring 8 represents essentially the stationarypart of the cam drive of the pump piston. While the device that movesthe pump pistons, which for example can be the ring 9 that guides theroller tappets 6, which ring is coupled to the drive shaft, representsthe moving part of the cam drive. The adjustment of the cam ring, whichis guided with its cylindrical outer wall in a corresponding cylindricalrecess 10 of the pump housing 11 of the fuel injection pump, produces anearlier or later running of the rollers 12 of the roller tappets 6 onthe respective cams 13, which are disposed so that all the rollertappets are moved synchronously inward or outward by the same strokes.As a result, the beginning of the feed stroke of the pump pistons, andthus the injection onset, changes in relation to the drive of the fuelinjection pump.

For adjustment purposes, the cam ring 8 has a nose 14 that engages in arecess 15 in an injection adjustment piston 16 on its cylindrical jacketface. The injection adjustment piston can be moved sealingly in acylinder 17 and with its one end face 18, together with the closed endof the cylinder 17, encloses a work chamber 20 and with its other endface 21 on the opposite end, encloses a spring chamber 22 in thelikewise closed cylinder there. A restoring spring 23 is disposed inthis chamber and is supported on one end against a wall 24 that closesthe cylinder 17 and on the other end against the end face 21 of theadjusting piston 16, and is consequently clamped so that it strives tobring the injection adjustment piston 16 with its one end face 18 intocontact with the wall 25 that closes the cylinder 17 on the oppositeend.

An axial blind bore is also provided in the injection adjustment piston16 as a cylinder bore 27 that guides a control slide valve 28 and openstoward the spring chamber 22. The control slide valve 28 inserted there,with one end face 29, encloses a control chamber 30 in the closed end ofthe cylinder bore and protrudes with its other end into the springchamber 22, where it is acted upon by a control spring 31, whose otherend is likewise supported against the wall 24. A connecting line 32leading from the one end face 18, extends in the control piston parallelto the cylinder bore 27 and feeds radially into the cylinder bore in theregion of its overlap by the control slide valve. This infeed can beclosed by an annular collar 33 of the control slide valve, wherein onboth ends, this annular collar defines a first annular groove 34 and asecond annular groove 35 and the first annular groove 34 communicateswith the control chamber 30 via a throttle 36 and the second annulargroove 35 communicates with the spring chamber 22 via an axial blindbore 37 as a pressure fluid discharge, which spring chamber is in turndischarged via a relief line 38. The annular edges that define theannular collar are control edges by means of which in a relativemovement of the control slide valve, the connecting line 32 is eitherconnected to a pressure fluid inlet 39 via the first annular groove 34or to the spring chamber 22 via the second annular groove 35. The firstannular groove 34 continuously communicates with the pressure fluidinlet 39, which is supplied with pressure fluid from a pressure storagechamber 40. A fuel pump 41 with a pressure control valve 42 connected inparallel is used to supply the storage chamber, and together, theysupply the pressure storage chamber in a known manner with a pressurethat essentially increases in a speed dependent manner with theincreasing speed of the fuel injection pump or the associated internalcombustion engine. This pressure also prevails continuously in thecontrol chamber 30 is such a way that according to the changing of thispressure, the control slide valve is slid in relation to its controlspring and this is then compressed or released. The annular collar 33carries out controlling functions in such a way that in an adjustment ofthe control slide valve to the right that occurs with increasingpressure, the work chamber 20 is supplied with pressure fluid until, asa result of a subsequent movement of the adjustment piston counter tothe force of the spring 23, the connecting line 32, which was previouslyopen, is closed once more. Conversely, with a reduction of the pressurein the control chamber 30, the work chamber 20 is discharged until theconnecting line is closed once more. The pressure in the control chamber30 can additionally be discharged or modified by means of a relief line43 in which an electrically controlled valve 44 is inserted.

In the known device for adjusting the onset of injection, the problemarises that reaction forces are transmitted to the cam ring 8 via therollers 12 when the pump pistons are driven, which forces are in turnpassed on to the injection adjustment piston 16 in such a way that thepressure in the work chamber 20 is abruptly increased within in thecompressibility frame of the fuel there, which is used here as pressurefluid. This pressure then is also present at the annular collar 33, viawhich due to leakage flows, fuel can also get into the storage chamber30 and leads to pressure increases there, which have repercussions onthe position of the control slide valve. In other types of distributorpumps that have a pump piston driven to reciprocate and simultaneouslyrotate, the fuel is typically supplied from the same pressure storagechamber 40 from which the control pressure for the control chamber istaken. Since in these pumps, the pressure storage chamber 40 is likewiseused as a fuel supply chamber and relief chamber of the high pressurepart, in particular the diversion processes at the end of the highpressure feed stroke here have a negative impact on the constancy of thecontrol pressure. This leads respectively to an unstable behavior of theinjection adjustment piston or of the entire device for adjustment ofthe onset of injection.

In the embodiment according to the invention, a pin 46 now protrudes onthe end with the end face 29 of the control slide valve 28, which pin,from the circular end face of the control slide valve in the prior art,now constitutes an annular end face 29. The pin protrudes through thecontrol chamber 30 and dips into a blind bore 48 that leads coaxiallyfrom the end of the cylinder bore 27 and encloses a damping chamber 49there on its end face. The outer diameter of the pin and the innerdiameter of the blind bore are embodied so that a throttle gap 50 isconstituted between the wall of the blind bore and the jacket face ofthe pin, via which gap fuel can overflow from the damping chamber 49into the control chamber 30 when the control slide valve or the pin 46is slid into the blind bore 48. This produces a very significant dampingof the control slide valve movement in such a way that when pressurefluctuations still occur in the control chamber 30, or when there areother forces acting on the control slide valve 28, the resultingmovement is sharply braked, damped, or even prevented.

The foregoing relates to preferred exemplary embodiments of theinvention, it being understood that other variants and embodimentsthereof are possible within the spirit and scope of the invention, thelatter being defined by the appended claims.

What is claimed and desired to be secured by Letters Patent of the United States is:
 1. A fuel injection pump having an injection adjustment piston (16) that is used to adjust an onset of injection and defines a work chamber (20) in a cylinder (17), said chamber is acted upon by a controllable pressure fluid, which forces the injection adjustment piston counter to a restoring spring (23), a control slide valve (28) is disposed so that the control slide valve can slide in a closed-ended cylinder bore (27) of the injection adjustment piston (16), in an axial direction of the injection adjustment piston (16), said control slide valve is adjusted counter to a force of a control spring (31) by a control pressure that prevails in a control chamber (30) enclosed inside the cylinder bore by the control slide valve and acts upon an end face (29) of the control slide valve, said control slide valve in the cylinder bore 27 includes control edges, said control edges control a pressure fluid inlet (39) into the cylinder bore (27), a pressure fluid outlet (37) from the cylinder bore, and a connecting line (32) from the cylinder bore (27) to the work chamber (20), wherein the pressure fluid inlet communicates via a throttle (36) with the control chamber (30) defined by the end face (29), and the end face (29) is embodied as an annular end face , which encompasses a pin (46) that protrudes from the control slide valve and extends into a blind bore (48) that adjoins the cylinder bore (27) coaxially on the inner end of the cylinder bore to form a throttle gap (50) between the wall of the blind bore and the jacket face of the pin. 